The Tories yesterday admitted breaking strict electoral rules over donations to the party.

They promised an investigation into cash handed over by millionaire Zac Goldsmith and other rich backers.

A series of five gifts totalling £38,950 between 2005 and 2008 were registered with the Electoral Commission as coming from a company called Unicorn Administration.

But it has been revealed the money actually came from a number of wealthy donors including Mr Goldsmith, now the party's candidate for Richmond Park. Other donors are said to include Mr Goldsmith's brother Ben and billionaire property developers the Reuben brothers. And while party bosses were apparently informed of the specific sources, they were not registered as such with the commission.

A Tory spokesman said: "It appears that there may have been an administrative error. We are taking immediate steps to investigate and rectify the declarations.

"Clearly, there was no motive of concealment. All those Unicorn was acting for have been declared as donating to the party."

But Lib Dem Treasury spokesman Lord Oakeshott said: "Back-door gifts through front companies blatantly break the spirit of the law on cleaning up Britain's moneybags politics. If the Conservatives have nothing to hide, why not declare straight out they're Goldsmith's thousands instead of using the cover of Unicorn Administration?

"It is hard to see how these can be simple errors."

A spokesman for Mr Goldsmith said his name was "clearly on the cheque" given to the Conservative party. He added: "The error was in no sense his."

£39k Tories failed to register who was behind almost £39,000 of donations by a firm